The present disclosure relates generally to a design methodology for a design layout, and particularly to a method of designing shapes for a design level employing sidewall image transfer within a design layout, and a method of generating a set of lithographic masks from the design layout for a sidewall image transfer process, and an apparatus for implementing the methods.
The sidewall image transfer process is a process employed to overcome the limitations of conventional lithography and to enable printing of sublithographic features employing conventional lithography tools. FIG. 1 illustrates the steps employed in a prior art sidewall image transfer process. A design layout including a design level is generated. The design layout includes a periodic one-dimensional array of design shapes, which are classified into two types. A first lithographic mask including every other design shapes and a dummy design shape is generated. Mandrel structures are formed on a physical structure (which can be a substrate) employing the first lithographic mask. Spacers are formed around the mandrel structures by deposition of a conformal material layer and an anisotropic etch. The mandrel structures are subsequently removed. A second lithographic mask is generated to block portions of the spacers overlying regions in which design shapes are not present in the design layout. A photoresist is applied over the physical structure, and cover regions corresponding to the block shapes in the second lithographic mask. A composite pattern derived from the shapes of the spacers and the second lithographic mask can be transferred into the physical structure.
While the sidewall image transfer process is conceptually simple, generation of the shapes to be included in the first lithographic mask and the second lithographic mask becomes exponentially difficult when the periodic one-dimensional array of design shapes is replaced with a non-periodic pattern including arbitrary design shapes. This is because the pattern that is transferred into a physical structure is not a direct combination of design shapes, but is a combination of shapes of spacers derived from a subset of the design shapes that are selected to allow formation of spacers therearound. Selection and alteration of design shapes around which spacers are to be formed provide astronomical numbers of possibilities. Difficulty in generation of the shapes to be included in the first lithographic mask and the second lithographic mask can arise to an unmanageable level if lateral interconnections are employed along non-lengthwise directions of the design shapes. Thus, a systematic method is desired that can reliably provide a working decomposition method for complex design layouts.